1. Field of the Invention
The present invention relates to an organic positive temperature coefficient thermistor and a manufacturing method therefor.
2. Related Background Art
A positive temperature coefficient (PTC) thermistor has a composition which comprises at minimum a pair of electrodes positioned facing each other and a thermistor element positioned between this pair of electrodes. Moreover, this thermistor element has a “positive temperature coefficient of resistance,” meaning that within a specific temperature range, its resistance rises sharply as the temperature rises.
Taking advantage of these features, positive temperature coefficient thermistors (hereunder “PTC thermistors”) are used for example as self-regulating heat generators, temperature sensors, current limiting elements, over-current protection elements and the like. For purposes of use as an over-current protection element in particular, a PTC thermistor needs to have low room-temperature resistance when not in operation, a large rate of change from room temperature resistance when not in operation to resistance when in operation, a small change in resistance when operated repeatedly (difference between resistance upon initial use and resistance after repeated operation), excellent breaking characteristics and a low heating temperature of the element, and it must be capable of being made small, light-weight and at low cost.
Conventional PTC thermistors have generally been of the type equipped with a thermistor element made of ceramic material, but this type of PTC thermistor has high room-temperature resistance and a high heating temperature of the thermistor element, and has been difficult to make small, light-weight and at low cost.
Therefore, in order to meet the aforementioned demand for lower operating temperature, lower room-temperature resistance and the like, a type of organic positive temperature coefficient thermistor is being studied which comprises a molded element consisting of a polymer matrix and conductive particles as the thermistor element (hereunder, “P-PTC thermistor”).
For example, a P-PTC thermistors of this sort has been proposed which is equipped with a thermistor element formed using low-density polyethylene as the polymer matrix and carbon black as the conductive particles (conductive filler) (see for example U.S. Pat. Nos. 3,243,758 and 3,351,882). The operating temperature of this thermistor element can be reduced by selecting an appropriate polymer matrix.
However, although such a P-PTC thermistor using carbon black as the conductive particles has lower room-temperature resistance than the aforementioned thermistor using a thermistor element made of ceramic material, it is becoming clear that its characteristics are still inadequate. Namely, it has been shown that if the conductive filler (carbon black) content is increased in an effort to reduce room-temperature resistance, the difference in resistance (rate of change in resistance) between the non-operating and operating states is reduced, and the thermistor cannot withstand actual use.
Therefore, these inventors and others have proposed P-PTC thermistors using nickel powder or other spiky particles as the conductive filler. Since the room-temperature resistance of such a P-PTC thermistor can be made sufficiently low, and the rate of change in resistance is high, the aforementioned problems of conventional PTC thermistors can be resolved. Moreover, it has been shown that it is also possible to reduce the operating temperature by appropriate selection of the matrix material as necessary, and that addition of a low molecular weight organic compound is effective as a method therefor.